Conventional lithographic printing techniques cannot accommodate true high-speed variable data printing processes in which images to be printed change from impression to impression, for example, as enabled by digital printing systems. The lithography process is often relied upon, however, because it provides very high quality printing due to the quality and color gamut of the inks used. Lithographic inks are also less expensive than other inks, toners, and many other types of printing or marking materials.
Ink-based digital printing uses a variable data lithography printing system, or digital offset printing system, or a digital advanced lithography imaging system. A “variable data lithography system” is a system that is configured for lithographic printing using lithographic inks and based on digital image data, which may be variable from one image to the next. “Variable data lithography printing,” or “digital ink-based printing,” or “digital offset printing,” or digital advanced lithography imaging is lithographic printing of variable image data for producing images on a substrate that are changeable with each subsequent rendering of an image on the substrate in an image forming process.
For example, a digital offset printing process may include transferring radiation-curable ink onto a portion of an imaging member (e.g., fluorosilicone-containing imaging member, imaging blanket, printing plate) that has been selectively coated with a dampening fluid layer according to variable image data. According to a lithographic technique, referred to as variable data lithography, a non-patterned reimageable surface of the imaging member is initially uniformly coated with the dampening fluid layer. Regions of the dampening fluid are removed by exposure to a focused radiation source (e.g., a laser light source) to form pockets. A temporary pattern in the dampening fluid is thereby formed over the printing plate. Ink applied thereover is retained in the pockets formed by the removal of the dampening fluid. The inked surface is then brought into contact with a substrate at a transfer nip and the ink transfers from the pockets in the dampening fluid layer to the substrate. The dampening fluid may then be removed, a new uniform layer of dampening fluid applied to the printing plate, and the process repeated.
Digital printing is generally understood to refer to systems and methods of variable data lithography, in which images may be varied among consecutively printed images or pages. “Variable data lithography printing,” or “ink-based digital printing,” or “digital offset printing” are terms generally referring to printing of variable image data for producing images on a plurality of image receiving media substrates, the images being changeable with each subsequent rendering of an image on an image receiving media substrate in an image forming process. “Variable data lithographic printing” includes offset printing of ink images generally using specially-formulated lithographic inks, the images being based on digital image data that may vary from image to image, such as, for example, between cycles of an imaging member having a reimageable surface. Examples are disclosed in U.S. Patent Application Publication No. 2012/0103212 A1 (the '212 Publication) published May 3, 2012 based on U.S. patent application Ser. No. 13/095,714, and U.S. Patent Application Publication No. 2012/0103221 A1 (the '221 Publication) also published May 3, 2012 based on U.S. patent application Ser. No. 13/095,778. These applications are commonly assigned.
Digital offset printing inks differ from conventional inks because they must meet demanding rheological requirements imposed by the variable data lithographic printing process while being compatible with system component materials and meeting the functional requirements of sub-system components, including wetting and transfer where the imaging member surface supports an image that is only printed once and is then refreshed. Each time the imaging member transfers its image to the print media or substrate, all history of that image remaining on the imaging member surface must be eliminated to avoid ghosting. Inevitably some film-splitting of the ink occurs at the transfer nip such that complete ink transfer to the print media cannot be guaranteed as residual ink may remain. This problem is a long felt need in the digital offset printing industry, with these systems requiring cleaning subsystems after the transfer nip to continuously remove post transfer residual ink from the reimageable surface of the imaging member prior to formation of the next print image. The inventors, aided by careful empirical testing and materials analysis, found and prescribe specific materials and system layout guidelines for more efficient and effective residual ink removal.